1. Field of the Invention
The present invention relates to preparation of trenches for laying underground pipelines and more particularly, to efficient preparation of trenches for laying underground pipelines.
2. Description of the Related Art
Typically, underground pipelines are used to deliver oil and gas products and other products, e.g., coal slurries and water, in the world-wide pipelining industry. Pipelines often must be laid in remote or undeveloped areas. Also, frequently pipelines must be laid in climates with extreme weather, such as Alaska. These soil conditions may make construction of pipelines difficult.
Trenches are dug to allow the pipelines to be placed underground, thus aiding in protecting them from the environment and avoiding obstructions such as rivers, bridges, railroad tracks, etc. Typically, when a pipeline is to be laid, a machine excavates a trench, delivering the excavated soil and rocks to one side of the trench, the excavated material forming a row of "spoil" along the side of the trench. The pipeline requires padding along the base of the trench to protect against damage from rough, sharp or pointed rocky materials that may be present along the lower portion of the trench. After placement of the pipeline, another layer of padding material is added on top of the pipeline to protect it further.
Care must be taken to surround the pipeline with trench filling that will not damage the pipeline. The pipelines are also covered with a cathodic coating to protect the pipe from rust and corrosion. Rocky material may damage the coating of the pipeline. Thus, the "padding" that constitutes the trench filling must be a fine material such as sand or fine soil that will maintain the integrity of the pipe.
As is known in the art, the padding material may be shipped to the pipeline construction area and then used to cushion the bottom of the trench and as protective filling around the pipe. The problem with this approach is that it is usually quite costly to transport the padding material. The distance for transport is usually considerable and the terrain may make transportation difficult. Also, there is an added labor cost for transporting the padding material.
In an attempt to lower the cost of transporting padding material to the pipeline construction site, in some instances, pulverizing machines have been conveyed to the construction site. Material from the site of construction was typically pulverized, screened to remove larger particles, and then used for padding material.
U.S. Pat. Nos. 3,701,422 and 4,633,602 relate to machines that move along the trench, picking up and screening the excavated material, and placing it on a longitudinally extending, rearwardly inclined conveyor that deposits the screened material on top of the pipeline as cover padding. Unfortunately, simply retrieving and screening the material that is excavated often does not provide sufficient material to pad the trench adequately.
However, the prior art techniques fail to maximize extraction of padding material from the excavated material. There is a need to provide an apparatus and method for obtaining a maximal amount of padding material from the excavated material so that a more efficient, cost effective process is achieved.